This invention relates generally to filter press membrane electrolytic cells. More specifically, it relates to a method for replacing membranes in a multiple unit filter press membrane electrolytic cell.
Chlorine and caustic, products of the electrolytic process, are basic chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chloralkali cells. The chloralkali cells today are generally of two principal types, the deposited asbestos diaphragm-type electrolytic cell or the flowing mercury cathode-type.
Comparatively recent technological advances, such as the development of dimensionally stable anodes and various coating compositions, have permitted the gap between electrodes to be substantially decreased or eliminated entirely. This has dramatically increased the energy efficiency during the operation of these energy-intensive units.
The development of a hydraulically impermeable membrane has promoted the advent of filter press membrane chloralkali cells which produce a relatively uncontaminated caustic product. This higher purity product obviates the need for caustic purification and concentration processing. The use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. However, continual advances have been made in the development of monopolar filter press membrane cells.
The use of a hydraulically impermeable membrane, however, presents problems should the membrane become structurally damaged, such as ruptured by the passage of a sharp object therethrough. Since commercial size filter press membrane cells comprise multiple cathode and anode units separated by a membrane, there may be up to twenty four or more membranes in each electrolytic cell unit. The exact position of a structurally damaged membrane in a electrolytic cell unit employing multiple membranes is difficult to identify without taking apart the entire filter press cell.
Typically, structural damage to one or more membranes manifests itself in several symptomatic ways. Cathode current efficiency and anode current efficiency decrease when a membrane is damaged. The cathode current efficiency decreases are detectable, such as by physically measuring the weight of the caustic produced in a container vessel and then calculating the production rate of caustic or by physically measuring the flow rate with appropriate means, for example flow totalizer units. The production rate of caustic is calculated by measuring the equivalents of caustic produced per current load and is measured in grams per gram equivalent.
The decrease in anode current efficiency is detectable because of an increase in the presence of oxygen and oxychlorides, such as hypochlorite, or chlorates, in the cell gas and the spent anolyte stream (spent brine). A change in the pH of the spent anolyte stream can also be an indicator of a decrease in anode current efficiency. The increase in the presence of oxygen may be determined by gas chromatograph testing, while the increase in the presence of oxychlorides can be detected by titration. The oxygen and oxychlorides are present because the caustic crosses through the membrane at the point of structural damage in back migration and starts to electrolyze or chemically react with the bulk anolyte. This puts hydroxyl ions back into a low pH environment which, depending on the type of anodes being used, will produce either oxygen, chlorite ions or chlorate ions.
Previously, when testing such as this detects the presence of decreased cathode current efficiency or decreased anode efficiency, the exact location of the structurally damaged membrane could be determined only by trial and error. This required that the entire electrolytic cell be taken apart and the anodes and cathodes be separated individually to check each membrane visually for structural damage. The entire process, including the diagnosis of a problem by the detection of a reduction in the cathode current efficiency or anode current efficiency and the breaking apart of the cells to find the damaged membrane or membranes could well take several days and up to a week. A loss of this much operating time for an electrolytic cell unit is costly and the steps necessary to correct the problem in this manner are labor intensive.
Once the location of the structurally damaged membrane is thusly identified, the practical problem of removing a single membrane in a multiple electrode unit filter press membrane cell, containing as many as 24 or more membranes, presents itself. Any shifting of the stack of electrodes with the membranes sandwiched between each pair of electrode frames during the cell disassembly and assembly can damage the membranes. This damage normally will be in the form of a tear to the membrane. Gasket misalignment could also occur, affecting the fluid-tight configuration of the assembled cell. Naturally, damage to additional membranes adds to the cost of the cell and requires more "down" or non-operating time to replace the damaged membranes.
Time consuming and labor intensive disassembly and assembly steps further compound the problem of replacing membranes. Prior approaches comprised breaking the entire cell apart by individually removing each electrode frame and membrane from the cell stack and replacing each electrode frame and membrane individually in the cell stack. The cell must first be disconnected from the electrical and electrolyte circulation circuits and then moved to a suitable disassembly area.
Where the entire cell is broken apart, each individual gasket adjacent each electrode frame must also be replaced to ensure a fluid-tight seal between the frames when the cell is reassembled. This is also a time consuming and labor intensive procedure.
Additionally, the prior approach would frequently damage more membranes than the originally damaged one because the membrane fibers would adhere to the surface of the gasket, whether gum, rubber or other material. This would result in the membranes tearing when the electrode frame with its attached gasket was removed from the stack.
The foregoing problems are solved by the improved method of replacing a structurally damaged membrane after determining the location of the damaged membrane in a multiple unit filter press membrane electrolytic cell in response to cell operating conditions and monitorings indicating the existence of a problem.